Owing to the high corrosion resistance, weldability, and coatability thereof, hot-dip galvanized steel sheets are widely used as automotive steel sheets. Automobile panels and structural members are required to have high strength for the purposes of weight reduction, improvements in fuel efficiency, and passenger safety, and thus many kinds of high-strength automotive steels have been developed.
Although steels for automobiles are required to have high formability for press-forming or roll-forming processes in addition to having high strength, most kinds of steel do not satisfy such requirements, and thus different kinds of steel are used according to individual applications. For example, dual-phase steel (DP steel in which relatively hard martensite formed by phase transformation is finely dispersed within a matrix of ferrite) having uniform ductility and deep drawing properties as well as a high degree of strength, owing to a low yield strength ratio and a high strain hardening rate, is commonly used in applications requiring press-forming processes, and martensite single-phase steel having a high degree of strength and a degree of balance between strength and bendability is used in applications requiring roll-forming processes.
Therefore, a large amount of research has recently been conducted into steel having a high degree of formability for both press-forming and roll-forming processes as well as having a high degree of strength. As a result, as disclosed in Patent Documents 1 to 4, austenitic high manganese steel has been proposed, in which manganese is included in an amount of 5% to 35% to induce the formation of twins during plastic deformation and to thus obtain a high degree of strength and a markedly improved degree of ductility.
However, since there is a limit to increasing the tensile strength of austenitic high manganese steel to 1200 MPa or greater, it is difficult to manufacture ultra-high-strength steel from austenitic high manganese steel. That is, alloying elements such as manganese (Mn), silicon (Si), chromium (Cr), or vanadium (V) have to be additionally added in order to obtain a degree of strength of 1200 MPa or greater.
However, if alloying elements are added to such high manganese steel, the hot-dip galvanization properties of the high manganese steel are further deteriorated. Specifically, when a hot-dip galvanized steel sheet is produced by coating a high manganese steel sheet through a hot-dip galvanizing process, an annealing process is usually performed under a nitrogen atmosphere including hydrogen so as to obtain the necessary material properties and surface activation (reduction) properties. In this case, the atmosphere serves as a reducing atmosphere for iron (Fe) in the high manganese steel sheet and an oxidizing atmosphere for easily oxidizable elements of the high manganese steel such as manganese (Mn), silicon (Si), aluminum (Al), or chromium (Cr). Therefore, if a high manganese steel sheet in which alloying elements such as aluminum (Al) and silicon (Si), as well as manganese (Mn), are included in large amounts is processed through a recrystallization annealing process under such an atmosphere, the alloying elements may be selectively oxidized (selective oxidation) by moisture or oxygen contained in small amounts in the atmosphere, and thus oxides such as oxides of manganese (Mn), aluminum (Al), or silicon may be formed on the surface of the high manganese steel sheet (base steel sheet to be coated). Therefore, when the high manganese steel sheet is coated, the high manganese steel sheet may not be properly coated, or a coating layer formed on the high manganese steel sheet may be removed in a later machining process.
That is, if alloying elements are added to high manganese steel sheets, it may be difficult to coat the high manganese steel sheets under general processing conditions. Furthermore, the addition of relatively expensive alloying elements may increase manufacturing costs.